We have been attempting to isolate and purify the specific endogenous mitotic inhibitor for human lymphocytes in vitro from aqueous extracts of lymphoid tissue from various animals. Our techniques so far have included the aqueous extraction at neutral pH of acetone powders of spleen or thymus which are subjected to molecular filtration, using an Amicon Diaflo system containing a 50,000-dalton filter. All the material that can move through a 50,000-dalton filter is then concentrated and dialyzed in another Diaflo chamber, using a membrane of 30,000 daltons. All the material that will not move through a 30,000- dalton filter is then collected and lyophilized. This starting material contains a specific endogenous mitotic inhibitors for lymphocyte transformation, whether it be stimulated by mitogen, antigen, or spontaneously, as is the case of cells derived from patients with lymphocytic leukemia. This extract has no effect upon the mitotic rate of other cell lines in culture, and extracts of non-lymphoid tissue do not demonstrate this effect, other than by cytotoxicity, upon lymphocytes' transformation in culture. This lymphocyte chalone is being further purified by, firstly, chromatography upon Con A-Sepharose and, secondly, preparative acrylamide gel electrophoresis. It is apparently a glycoprotein because of its solubility in trichloracetic and perchloric acids and in ethanol, and since it binds to Con A it must contain terminal mannose. It is thermolabile and destroyed by treatment with trypsin.